Self-organized quantum dot manufacturing method and quantum dot semiconductor structure

ABSTRACT

The invention provides a quantum dot manufacturing method and related quantum dot semiconductor structure. The quantum dot semiconductor structure includes: a conductive ridge on a substrate; an insulative layer covering the substrate and the conductive ridge, wherein the insulative layer includes a top portion and two sidewalls over the conductive ridge; a plurality of quantum dots respectively embedded within a plurality of silicon dioxide spacer islands, which are adhered to the sidewalls of the insulative layer; and a plurality of conductive ledges adhered to the silicon dioxide spacer islands, wherein each of the conductive ledges is a portion of an electrode with alignment to the corresponding quantum dot.

BACKGROUND OF THE INVENTION Field of Invention

The present invention relates to a quantum dot manufacturing technology, especially a quantum dot manufacturing technology having tunable and self-aligned capability, and compatibility with CMOS manufacturing process.

Description of Related Art

Recently, semiconductor quantum dots (can be abbreviated as QDs) qubits have emerged as the subject of intensive research. The excellent properties of quantum dots have caused researchers and companies to consider using them in several fields for future development.

Quantum dots are a few nanometers in size and their sizes are very critical for their electrical and optical properties. Quantum dots have electronic properties that differ from high dimensional materials due to quantum confinement. The prior quantum dot manufacturing processes can be based on chemical synthesis, wherein the quantum dots have good size tunability, but the placement precision of quantum dots (especially the inter-dot spacing between quantum dots for quantum entanglement in quantum computing) is difficult, presenting a major challenge in making electrical contacts to specific QDs. Or, the quantum dots can be formed by epitaxial growth, wherein the placement precision of quantum dots is still difficult to achieve. Or else, lithography is another option for forming the quantum dots, but producing lithographically-patterned quantum dots with sufficiently small sizes and inter-dot spacings are difficult in terms of uniformity and reproducibility due to the resolution of lithography techniques (the current size resolution is about 10 nanometers by EUV). In short, the prior quantum dot manufacturing methods are difficult for obtaining sufficiently small, closely-coupled quantum dots with placement precision, to let alone how to tune the size of (and spacing between) quantum dots in the manufacturing process.

Further, under the quantum dot nanometer size and placement requirement, after forming and placing the quantum dots, making electrical contacts to the quantum dots is also a challenge in such a high placement precision requirement. Further, when nanoscale, closely-coupled quantum dots are formed laterally and vertically, making electrodes to specific quantum dots not only one-dimensional, but multiple-dimensional, which greatly increase the difficulty.

Besides the aforementioned problems of the prior quantum dot technology, the operation temperature of quantum dot devices, in particular for quantum computing devices, is another challenge. To date, the operation of quantum-dot qubits can be validated at a very low temperature (for example, lower than 2 K). This operation temperature requirement is strongly related to the sizes of and spacing between the quantum dots.

SUMMARY OF THE INVENTION

To the technical problems above-mentioned, the objects of the present invention are to provide a quantum dot manufacturing method and related quantum dot configuration. The quantum dots produced by this method can be tuned in terms of size and inter-dot spacing using existing CMOS manufacturing processes. Due to this high controllability and reproducibility, these quantum dots are suitable for large-scale quantum computing device fabrication. The semiconductor structure of the present invention in principle can perform quantum transport at relatively high temperatures (>100 K or even room temperature). Further, the sizes of the quantum dots can be downsized to 5 nanometers or less. Importantly, the quantum dot manufacturing method of the present invention is compatible with the existing CMOS manufacturing process, without need for other special devices and processes for making the quantum dots.

In one perspective, the present invention provides a quantum dot manufacturing method, which includes: forming a conductive ridge on a substrate; forming an insulative layer covering the substrate and the conductive ridge, wherein the insulative layer is coated on the conductive ridge to conformally encapsulate the conductive ridge; forming a semiconductor-alloyed layer over the insulative layer, and etching back the semiconductor-alloyed layer to form symmetrical, paired semiconductor-alloyed spacer islands separately at the sidewall edges of the insulative layer; and forming symmetrical quantum dots and their cladding-layers of silicon dioxide through thermal oxidation of the semiconductor-alloyed (such as SiGe-alloyed) spacer islands, wherein the Si content is preferentially oxidized forming cladding silicon dioxide and the Ge content is segregated and then ripened for forming Ge quantum dots.

In another perspective, the present invention provides a paired, quantum dot configuration, which includes: a conductive ridge on a substrate; an insulative layer covering the substrate and the conductive ridge, wherein the insulative layer includes a top portion and two sidewalls over the conductive ridge; a pair of quantum dots respectively self-organizedly capped by silicon dioxide spacer islands, and the quantum dots adhered to the sidewall edges of the insulative layer; and a pair of conductive ledges adhered to the silicon dioxide spacer islands, wherein each of the conductive ledges is a portion of an electrode self-aligned to the corresponding quantum dot via the silicon dioxide spacer islands.

In order to better understand the above and other aspects of the present invention, the detailed description of the embodiments and the accompanying drawings are provided as follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1E show a quantum dot manufacturing method according to one embodiment of the present invention.

FIGS. 1F and 1G show a conductive ledge manufacturing process according to one embodiment of the present invention.

FIGS. 1H and 1I show a perspective view of a separated quantum dot semiconductor structure manufacturing process according to one embodiment of the present invention.

FIG. 2 shows a quantum dot semiconductor structure according to one embodiment of the present invention.

FIGS. 3A and 3B show various inter-dot spacing tuning statuses according to the present invention.

FIGS. 4A-4C show various quantum dot tuning statuses according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objectives, technical details, features, and effects of the present invention will be better understood with regard to the detailed description of the embodiments below, with reference to the drawings.

In one perspective, please refer to FIGS. 1A-1E, the present invention provides a quantum dot manufacturing method, which includes: forming a conductive ridge 10 (including a conductive material therein) on a substrate Sub (FIG. 1A); forming an insulative layer 20 to cover the substrate Sub and the conductive ridge 10 (FIG. 1B), wherein the insulative layer 20 is coated on the conductive ridge 10 to conformally form a top portion 21 on a top 11 of the conductive ridge 10 and two sidewalls 22 adhered to two sidewalls 12 of the conductive ridge 10; conformally forming a semiconductor-alloyed layer Lms over the insulative layer 20 (FIG. 1C), and etching back the semiconductor-alloyed layer Lms to form a plurality of semiconductor-alloyed spacer islands Sm separately adhered to the sidewalls 21 of the insulative layer 20 (FIG. 1D), wherein because of a height difference between the semiconductor-alloyed layer Lms on the top portion 21 and the semiconductor-alloyed layer Lms on a bottom planar area, the semiconductor-alloyed layer Lms adhered to the sidewalls 22 of the insulative layer 20 are etched into the semiconductor-alloyed spacer islands Sm after etching; and through thermal oxidation in the semiconductor-alloyed spacer islands Sm, forming a plurality of quantum dots 30 and a plurality of silicon dioxide spacer islands 40 (FIG. 1E), wherein the quantum dots 30 are gradually pushed inward (spacing S1 is getting shorter) during forming the silicon dioxide spacer islands 40.

In one embodiment, the quantum dot manufacturing method further includes: forming a conductive layer Lc over the silicon oxidation spacer islands 40 and the insulative layer 20 (FIG. 1F), and etching back the conductive layer Lc to form a plurality of conductive ledges 50 adhered to the silicon dioxide spacer islands 40 (FIG. 1G), wherein each of the conductive ledges 50 can be used a portion of an electrode with self-alignment to the corresponding quantum dot 30. With these silicon dioxide spacer islands 40, the electrodes can have very good alignments to the quantum dots, which solved the prior problem of poor alignment between the electrodes and the quantum dots. Regarding the forming of the conductive ledges 50, please refer to the aforementioned explanation of forming the semiconductor-alloyed spacer islands Sm, for the detail of forming the conductive ledges 50.

In one embodiment, when the semiconductor-alloyed layer Lms includes a silicon-germanium alloy (Si_(1-x)Ge_(x)), wherein a molecular ratio (X) means a proportion of germanium in the silicon-germanium alloy. In the thermal oxidation, germanium is segregated in the silicon-germanium alloy to form germanium quantum dots; and silicon therein is simultaneously oxidized, to form silicon dioxide spacer islands 50 respectively adhered to the two sidewalls 22. During the thermal oxidation, volumes of silicon dioxide spacer islands 40 grow such that the quantum dots 30 are gradually pushed inward due to gradually larger volumes of the silicon dioxide spacer islands 40. Besides, when a molecular ratio (X) of germanium exists in the silicon-germanium alloy is higher (lower), and/or the semiconductor alloyed spacer islands have higher (less) Ge contents, the sizes of the formed germanium quantum dots 30 can be bigger (smaller). Therefore, according to this principle, the sizes of germanium quantum dots 30 can be controlled and tunable.

In the aforementioned embodiment, the germanium quantum dots are used to illustrate the feature of the present invention. However, the metal can be not limited to germanium, such as, GeSn, SiC, GaAs, and so on.

Please refer to FIG. 1G, in one embodiment, with reference to the conductive ridge 10, wherein the quantum dots 30 on two sides of the conductive ridge are symmetric. This “symmetric” is a very important feature for quantum computing, because it can provide high and controllable quantum entanglement performance. Further, the “symmetric” in the present invention includes not only “mirror symmetric”, but other symmetric possibilities; for example, rotational symmetry, etc.

In one embodiment, a material of the insulative layer can include: silicon nitride, titanium nitride, or other Si-based insulative materials.

Please refer to FIG. 1, in the present invention, the substrate Sub can be a base structure for forming various semiconductor structure thereon. The substrate can include a silicon base Sb and a dielectric layer Ld, wherein a material of the dielectric layer Ld can include silicon dioxide. The quantum dot semiconductor structure of the present invention can be formed on the dielectric layer Ld, the silicon base Sb, or the silicon base Sb with another insulation means in between the quantum dot semiconductor structure and the silicon base Sb.

In one embodiment, a material of the conductive ridge includes poly-silicon.

In one embodiment, the step of forming semiconductor-alloyed spacer islands Sm adhered to the sidewalls 22 of the insulative layer 20, further includes: forming a photoresist layer in a mask pattern of a plurality of strips over the top portion 21, the sidewalls 22 and the semiconductor-alloyed spacer islands Sm (FIG. 1H); and photo etching the top portion 21, the sidewalls 22 and the semiconductor-alloyed spacer islands Sm according to the mask pattern, to form a plurality of groups with separated top portions, separated sidewalls, and separated semiconductor-alloyed spacer islands (FIG. 1I). In a top view direction (FIG. 1H), the aforementioned strips are substantially perpendicular to the extension direction of the conductive ridge 10. After the etching, the volumes of the separated semiconductor-alloyed spacer islands are respectively related to the sizes of germanium quantum dots in each group; that is, the strip widths of the separated semiconductor-alloyed spacer islands are related to the sizes of germanium quantum dots.

Please refer to FIG. 2, in quantum computing, an inter-dot spacing Sp between the quantum dots 30 is important for quantum entanglement and superposition behavior. In one embodiment, the width of the conductive ridge D can be used to control the inter-dot spacing Sp. Furthermore, a time period through the thermal oxidation process can be also used to control the inter-dot spacing Sp. For example, when the time period is longer, the volumes of silicon dioxide spacer islands grow further such that the quantum dots 20 are gradually pushed inward (shorter Sp), and the time period can be used to control the inter-dot spacing Sp.

In the present invention, the quantum dots 30 are formed by gradually reducing the germanium in the semiconductor (SiGe) alloy in the thermal oxidation, and pushed inward by the growth of the silicon dioxide spacer islands 40. In this process, the formed quantum dots 30 completely coalesce and each of the quantum dots has a single crystal structure, such that the quantum dots formed by the present invention have good quantum performance.

Please refer to FIG. 1G, in another perspective, the present invention provides a quantum dot semiconductor structure 100, which includes: a conductive ridge 10 on a substrate Sub; an insulative layer 20 covering the substrate and the conductive ridge, wherein the insulative layer 20 includes a top portion 21 and two sidewalls 22 over the conductive ridge 10; a plurality of quantum dots respectively disposed within a plurality of silicon dioxide spacer islands 40, and the silicon dioxide spacer islands 40 adhered to the sidewalls 21 of the insulative layer 20; and a plurality of conductive ledges 50 adhered to the silicon dioxide spacer islands 40, wherein each of the conductive ledges 50 is a portion of an electrode with alignment to the corresponding quantum dot 30.

Regarding the inter-dot spacing tuning, please refer to FIGS. 3A and 3B, wherein the inter-dot spacings are various between two sections of real quantum dot semiconductor structures with different widths of the conductive ridges. It is shown that the inter-dot spacing in the present invention can be controlled. Besides the ridge widths, the inter-dot spacing can be also controlled by the time period of the thermal oxidation process.

Regarding the quantum dot size tuning, please refer to FIGS. 4A, 4B, and 4C, wherein the quantum dot sizes are different among sections of several quantum dot semiconductor structures. It shows that the quantum dot sizes by the present invention can be various. According to the present invention, the control of quantum dot sizes can be done according to the time period of the thermal oxidation process, the volumes of the semiconductor-alloyed spacer islands, and/or the molecular ratio of germanium exists in the silicon-germanium alloy.

As shown in FIGS. 3A, 3B, 4A, 4B and 4C, the shapes of the manufactured quantum dots are spherical, and these shapes have the benefit of low surface energy. In comparison with the pyramid shape according to the prior epitaxial growth, the spherical shapes of the quantum dots provided by the present invention have a higher quantum property stability. Also, the electrostatic potential of a spherical quantum dot is radially symmetric, giving rise to the atom-like orbitals. The symmetry of this three-dimensional steep potential of a tiny quantum dot leads to a complete filling of orbitals of 1 s, 2 s, 2p, 3 s, and 3p for 2, 10, 18 electrons in sequence with very large addition energies, which is a powerful knob for controlling electron number when a quantum-dot device operating in the few-electron regime.

The present invention has been described in considerable detail with reference to certain preferred embodiments thereof. It should be understood that the description is for illustrative purpose, not for limiting the scope of the present invention. Those skilled in this art can readily conceive variations, combinations and modifications within the spirit of the present invention. 

What is claimed is:
 1. A quantum dot manufacturing method, comprising: forming a conductive ridge on a substrate; forming an insulative layer covering the substrate and the conductive ridge, the insulative layer forming a top portion and two sidewalls over the conductive ridge; forming a semiconductor-alloyed layer over the insulative layer, and etching back the semiconductor-alloyed layer to form a plurality of semiconductor-alloyed spacer islands adhered to the sidewalls of the insulative layer; and forming a plurality of quantum dots and a plurality of silicon dioxide spacer islands, through thermal oxidation in the semiconductor-alloyed spacer islands.
 2. The quantum dot manufacturing method of claim 1, wherein a material of the semiconductor-alloyed layer includes a silicon-germanium alloy (Si_(1-x)Ge_(x)).
 3. The quantum dot manufacturing method of claim 2, wherein the quantum dots are germanium quantum dots, and the sizes of the germanium quantum dots correspond to a molecular ratio (X) of germanium in the silicon-germanium alloy (Si_(1-x)Ge_(x)), and/or a plurality of heights of the semiconductor-alloyed spacer islands.
 4. The quantum dot manufacturing method of claim 1, wherein the quantum dots are symmetric with reference to the conductive ridge.
 5. The quantum dot manufacturing method of claim 1, wherein a material of the insulative layer includes silicon nitride.
 6. The quantum dot manufacturing method of claim 1, wherein the substrate includes a silicon base, and a dielectric layer on the silicon base.
 7. The quantum dot manufacturing method of claim 1, further comprising: forming a conductive layer over the silicon oxidation spacer islands and the insulative layer, and etching back the conductive layer to form a plurality of conductive ledges adhered to the silicon dioxide spacer islands, wherein each of the conductive ledges is a portion of an electrode with alignment to the corresponding quantum dot.
 8. The quantum dot manufacturing method of claim 1, wherein the step of forming semiconductor-alloyed spacer islands adhered to the sidewalls of the insulative layer, further comprising: forming a photoresist layer in a mask pattern of a plurality of strips over the top portion and the sidewalls of the insulative layer, and the semiconductor-alloyed spacer islands; and photo etching the top portion and the sidewalls of the insulative layer, and the semiconductor-alloyed spacer islands according to the photoresist layer, to form a plurality of groups with separated top portions, separated sidewalls, and separated semiconductor-alloyed spacer islands.
 9. The quantum dot manufacturing method of claim 1, wherein an inter-dot spacing between the quantum dots corresponds to a width of the conductive ridge, and/or a time period of the thermal oxidation.
 10. The quantum dot manufacturing method of claim 1, wherein the quantum dots are formed in the silicon dioxide spacer islands, or between the insulative layer and the silicon dioxide spacer islands.
 11. The quantum dot manufacturing method of claim 10, wherein a spacing between the electrode and the corresponding quantum dot, corresponds to a portion of the silicon dioxide spacer island between the electrode and the corresponding quantum dot. 